Standing Rock protesters celebrate move; fate of pipeline is uncertain, as Donald Trump has indicated support.

The Obama administration said Sunday that it had denied a permit needed to complete the last leg of an oil pipeline across the Midwest, prompting cheers from opponents but warnings that the move could be short-lived since President-elect Donald Trump supports the project. The nearly 1,200-mile Dakota Access pipeline, extending from North Dakota across parts of South Dakota and Iowa and ending in Illinois, is nearly complete, except for a 1,100-foot crossing of a Missouri River reservoir.

Protesters led by the Standing Rock Sioux Tribe have been gathering for months near Cannon Ball, N.D., close to the site of the crossing at Lake Oahe, and have argued that the pipeline endangers the tribe’s water supply and sacred sites. The pipeline’s builder, Dallas-based Energy Transfer Partners LP, has said it completed all necessary permitting requirements for the project and worked to minimize any damage to traditional sites and the risk of an oil spill.

On Sunday, the U.S. Army Corps of Engineers said it would deny the company an easement it needs for the $3.8 billion project. The agency also called for a full environmental review and re-evaluation of whether the pipeline’s route should be altered.

Sunday’s announcement comes nearly three months after the Army Corps said that it would review the project and engage in further talks with the Standing Rock Sioux to hear the tribe’s concerns. Energy Transfer has argued in federal court that the Army Corps had granted approval for the reservoir crossing in July. Pipeline experts said it was extremely rare for an administration to intervene in a permitting process typically handled by career civil servants. The advanced stage of the project’s construction made the Obama administration’s move even more unusual, and experts said they believed it could be easily overturned.

“Clearly this is being directed politically from the administration,” said Brigham McCown, a former top pipeline administrator for the Transportation Department during the George W. Bush administration. “What it says is that even if you have your permits in hand, the government might change the rules on you once construction is almost done.” Mr. McCown said Mr. Trump would have several options once he got in office, including directing the Secretary of the Army to reinstate a previously approved permit, or issuing an executive order stating that prior environmental assessments were adequate and approving the pipeline.

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